WO2020040019A1

WO2020040019A1 - Optical fiber loss measurement device and optical fiber loss measurement method

Info

Publication number: WO2020040019A1
Application number: PCT/JP2019/031969
Authority: WO
Inventors: 友和小田; 央高橋; 邦弘戸毛; 真鍋　哲也
Original assignee: 日本電信電話株式会社
Priority date: 2018-08-22
Filing date: 2019-08-14
Publication date: 2020-02-27
Also published as: JP2020030106A; JP6927172B2; US20210310897A1; US11402295B2

Abstract

Provided is an optical fiber loss measurement device capable of accurately measuring loss for a prescribed mode at various positions on an optical fiber through which a plurality of modes can propagate. This optical fiber loss measurement device comprises a means for inputting pump light of a prescribed mode and a first frequency into one end of an optical fiber for measurement through which a plurality of modes can propagate and inputting probe light of a second frequency having a Brillouin frequency shift applied thereto into the other end, a Brillouin gain acquisition means for measuring the intensity of the light output from the one end and using BOTDA to acquire Brillouin gains in the longitudinal direction of the optical fiber for measurement, and a means for measuring the loss in the optical fiber for measurement for the prescribed mode by comparing the sizes of the Brillouin gains at various positions in the longitudinal direction of the optical fiber for measurement. The mode of the probe light is such that the electric field distribution at a cross section of the optical fiber for measurement is axisymmetric.

Description

Optical fiber loss measuring device and optical fiber loss measuring method

The present invention relates to an optical fiber loss measuring device and an optical fiber loss measuring method, and more particularly, to an optical fiber loss measuring device that non-destructively measures loss experienced by each propagation mode propagating in an optical fiber. And a method for measuring an optical fiber loss.

2. Description of the Related Art In recent years, with a rapid increase in transmission traffic, an FMF (Few mode fiber) or an MMF (Multi mode fiber) that can use a plurality of propagation modes instead of an SMF (Single mode fiber) used in a current transmission path has been developed. It has attracted great attention as a device capable of further increasing the capacity. In these optical fibers, crosstalk coupling into a plurality of modes occurs during propagation. Therefore, mode multiplex communication is realized by performing crosstalk compensation by signal processing on the receiving side.

On the other hand, if a loss difference between modes occurs due to different losses received for each mode, crosstalk cannot be compensated. Therefore, it is necessary to reduce the loss difference between modes in the transmission path. The loss difference between modes generally occurs in a device such as an EDFA (Erbium Doped Optical Fiber Amplifier) or a mode multiplexer / demultiplexer, or at a connection point of an optical fiber. That is, it is necessary to measure the loss for each mode in the longitudinal direction of the optical fiber.

The reason that the loss received for each mode is different is that the shape of the electric field distribution in the cross section of the optical fiber is different. Electric field distribution shows different shapes in the optical fiber cross-section for each mode, roughly divided into strength and axisymmetric mode (LP _0j mode), there are two types of non-axisymmetric mode (LP _ij mode). For the non-axisymmetric LP _ij mode, in addition to the shape of the propagation mode, there is also an angle pattern of the axis of the optical fiber cross section. At this time, for the non-axisymmetric mode, even if the mode is the same, the loss differs for each rotation angle of the mode (Non-Patent Document 1). Therefore, in these modes, it is necessary to change the rotation of the mode, such as by rotating the phase plate, and acquire the loss received at that time.

Until now, OTDR (Optical Time Domain Reflectometry) has been proposed as a device for measuring loss in the longitudinal direction of an optical fiber (Non-Patent Document 2). However, in OTDR, loss measurement is performed using Rayleigh scattering generated in an optical fiber. However, since multiple modes are generated in this scattering process, information is obtained as a mixture of multiple modes, and a pure loss for each mode is obtained. I can't get it. For example, when acquiring the loss of LP ₁₁ mode, if the general loss was mixed with a small LP ₀₁ mode than LP ₁₁ mode, the loss of LP ₁₁ mode are measured actually smaller than. As described above, the loss measurement method using the backscattered light generated in the optical fiber has a problem that the influence of the occurrence of a plurality of modes cannot be excluded and the loss for each mode cannot be correctly evaluated.

As described above, conventionally, the OTDR is mainly used to measure the loss for each mode. However, as a method for evaluating the propagation characteristics for each mode in an optical fiber in which a plurality of modes propagate, a Brillouin using stimulated Brillouin scattering is used. A gain analysis method (BOTDA: Brillouin \ Optical \ Time \ Domain \ Analysis) (Non-Patent Document 3) has been proposed. BOTDA is to observe the propagation characteristics of each mode in the longitudinal direction of the optical fiber by utilizing the fact that only a specific mode can be generated in the optical fiber by controlling the frequency difference between the pump light and the probe light incident on the optical fiber. Can be.

However, when the loss for each mode is measured using BOTDA, the Brillouin gain generated in the optical fiber is not only the loss of the mode, but also the polarization state of the mode and the overlap of the electric field distribution of pump light and probe light. As a result, the loss of each mode cannot be measured correctly. In this regard, since heretofore is the use of LP ₀₁ mode only the fundamental mode in the SMF, the overlapping of the electric field distribution of the pump light and the probe light is always the same, and for the polarization state, the polarization scrambler etc. Has been used to make the amount of gain generated polarization independent (Non-Patent Document 4).

On the other hand, the FMF and MMF several modes propagating, varying the polarization of the LP ₁₁ mode, simultaneously change the electric field distribution. In addition, since these modes involve changes such as the electric field distribution rotating in the azimuthal direction of the optical fiber cross section during propagation, the use of such modes for both pump light and probe light changes the state of overlap in the longitudinal direction. The problem of doing so also arises.

Thus, in an optical fiber in which a plurality of modes propagate, even if an attempt is made to measure the loss for each mode using BOTDA, which can evaluate the propagation characteristics for each mode, the loss for each mode and each mode rotation is determined from the generated gain. Can not be measured.

The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide an optical fiber in which a plurality of modes propagate, an optical fiber capable of accurately measuring a loss of a predetermined mode at each position of the optical fiber. An object of the present invention is to provide a fiber loss measuring device.

In order to solve the above-described problem, the optical fiber loss measuring device described in one embodiment transmits light of a first frequency in a predetermined mode to an optical fiber to be measured that propagates a plurality of modes. A light incidence unit that enters as pump light, and that emits, as probe light, light of a second frequency lower than the first frequency by a frequency corresponding to the Brillouin frequency shift of the predetermined mode as probe light; Brillouin gain acquisition means for measuring the intensity of the light output from the optical fiber and acquiring the Brillouin gain in the longitudinal direction of the optical fiber to be measured by the BOTDA (Brillouin Optical Time Domain Analysis) method; The magnitude of the Brillouin gain at each position in the longitudinal direction is compared. By measuring the loss of the optical fiber to be measured in the predetermined mode, and a means for measuring the loss of the optical fiber, the probe light, regardless of the mode of the pump light, the The electric field distribution in the section of the optical fiber to be measured is axially symmetric.

The loss measuring method of the optical fiber described in another embodiment, with respect to the optical fiber to be measured that propagates a plurality of modes, the light of the first frequency of the predetermined mode is incident as pump light, A light incidence step in which light having a second frequency lower than the first frequency by a frequency corresponding to the Brillouin frequency shift of the predetermined mode is made incident as probe light, and the intensity of light output from the optical fiber to be measured is Measuring and obtaining a Brillouin gain in a longitudinal direction of the optical fiber to be measured by a BOTDA (Brillouin Optical Time Domain Analysis) method; and obtaining the Brillouin gain in the longitudinal direction of the optical fiber to be measured in advance. By comparing the magnitude of Brillouin gains Measuring the loss of the optical fiber under test in the predetermined mode, wherein the probe light is the light under test irrespective of the mode of the pump light. The mode is one in which the electric field distribution in the cross section of the fiber is axially symmetric.

It is a figure showing an example of an optical fiber loss measuring device concerning a 1st embodiment. FIG. 4 is a diagram illustrating pump light and probe light at point A and their interaction. FIG. 5 is a diagram illustrating pump light and probe light at point B and their interaction. It is a figure showing an example of an optical fiber loss measuring device concerning a 2nd embodiment.

Hereinafter, embodiments of the present invention will be described in detail.

The optical fiber loss measuring apparatus according to the present embodiment receives pump light in a predetermined mode from one of the optical fibers and probe light in a mode in which the electric field distribution in the cross section of the optical fiber is axially symmetric from one or the other. By obtaining the Brillouin gain generated by doing, and comparing the magnitude of the Brillouin gain at each position in the longitudinal direction of the optical fiber, the loss in a predetermined mode in the optical fiber is measured in the longitudinal direction in a distributed manner. Is what you can do.

[First Embodiment]
FIG. 1 is a diagram showing an example of the optical fiber loss measuring device according to the first embodiment. The optical fiber loss measuring device according to the present embodiment shown in FIG. 1 is connected to a laser beam generating unit 11, a branch element 12 having an input side connected to the laser beam generating unit 11, and one of two branches of the branch element 12. Pulse generator 13 and frequency control means 14 connected to the other, mode demultiplexing means 15 connected to pulse generator 13, optical circulator 17 connected to mode demultiplexing means 15, optical circulator 17 The optical fiber f to be measured and the photoelectric conversion means 18 having one end connected to the A / D conversion means 19, the data extraction means 20, and the gain analysis means 21 connected in series to the photoelectric conversion means 18; A configuration including an axially symmetric mode conversion means 16 provided between the frequency control means 14 and the other end of the optical fiber f to be measured is shown.

In the optical fiber loss measuring device of the present embodiment, the light output from the laser light generating means 11 for generating coherent light is split into two by the splitting element 12. One of the two branched lights is a pump light p1, which is pulsed by a pulse generator 13 and then converted from a fundamental mode to a higher-order propagation mode corresponding to the optical fiber f to be measured by a mode branching means 15. Is incident on the optical fiber f to be measured.

The mode demultiplexing unit 15 can use a phase plate, a PLC, a spatial light modulator, or the like, and converts the mode of the pump light p1 into a propagation mode in which a loss is to be obtained. The mode demultiplexing means 15 can convert the pump light p1 into an axially symmetric mode or a non-axially symmetric mode, and make it incident on the optical fiber f to be measured via the optical circulator 17.

The mode demultiplexing unit 15 evaluates the loss characteristics of each mode rotation state in the cross-sectional direction (optical fiber cross-sectional direction) of the optical fiber f to be measured when the pump light p1 is converted into a non-axially symmetric mode. To make it possible, a non-axisymmetric mode rotation mechanism for rotating the electric field distribution in the cross section direction of the optical fiber may be provided.

The frequency control means 14 provides the probe light p2, which is the other of the branched lights, about 10 times corresponding to the Brillouin frequency shift corresponding to the mode combination of the pump light and the probe light incident on the optical fiber f to be measured. A frequency difference of about 11 GHz is provided. This frequency control means 14 uses two lasers having different frequencies (wavelengths) even when controlled by an external modulator such as an SSB modulator made of LiNb ₃ and controls the optical frequency difference between the two lasers. May be. As the Brillouin frequency shift corresponding to the combination of the modes of the pump light and the probe light, the Brillouin frequency shift amount in the mode of the pump light can be used. Specifically, the frequency of the probe light can be set to a frequency lower than the frequency of the pump light by a frequency corresponding to a Brillouin frequency shift in a predetermined mode.

The 対称 axis symmetric mode conversion means 16 converts the probe light p2 to which the frequency difference has been imparted into the axis symmetric propagation mode, and enters the optical fiber f to be measured from the opposite direction to the pump light p1. The axial symmetric mode conversion means 16 may have the same configuration as that of the mode demultiplexing means 15, and when the fundamental mode among the axial symmetric modes is used, the SMF and the FMF may be directly connected.

When the pump light p1 and the probe light p2 are incident from both ends of the optical fiber f to be measured, respectively, the optical circulator collides with the pump light p1 and the probe light p2 at each position in the longitudinal direction of the optical fiber f to be measured. Brillouin gain occurs in the probe light output toward 17. In FIG. 1, the probe light having received the Brillouin gain is shown as Brillouin scattered light s.

The optical circulator 17 sends the probe light p2 (Brillouin scattered light s) amplified by the pump light p1 to the photoelectric conversion means 18. The photoelectric conversion unit 18 converts the amplified light intensity of the probe light p2 (Brillouin scattered light s) into an electric signal, converts the signal into digital data by the A / D conversion unit 19, and then converts the signal intensity of the light by the data extraction unit 20. Is extracted, and the gain is analyzed by the gain analysis means 21 from the extracted data.

In the analysis of the Brillouin gain, the Brillouin gain in the longitudinal direction of the optical fiber f to be measured is obtained from the intensity of light output from one end of the optical fiber f to be measured by the BOTDA (Brillouin Optical Time Domain Analysis) method. Specifically, first, the signal intensity of the probe light extracted by the data extracting means 20 when the pump light is not incident is acquired as the reference intensity. Thereafter, the signal intensity extracted by the data extracting means 20 when the pump light and the probe light are incident is acquired. Further, the Brillouin gain can be obtained by calculating the amount of increase in the signal intensity from the reference intensity when the pump light and the probe light are incident. By this measurement, by comparing the magnitudes of the Brillouin gains at each position in the longitudinal direction of the measured optical fiber, it is possible to measure the loss in a predetermined mode at each position in the longitudinal direction of the measured optical fiber.

(Brillouin gain)
Here, the Brillouin gain generated in the optical fiber f to be measured will be described. Considering the stimulated Brillouin scattering phenomenon in FMF as an example, the Brillouin frequency shift ν _b in an arbitrary mode is given by Expression (1).

In the formula (1), n _i is the effective refractive index of the mode, the V _a effective speed of the acoustic wave, lambda is the wavelength.

According to equation (1), in the FMF, it means that the Brillouin frequency shift is different depending on the propagating mode, and the Brillouin spectrum information at an arbitrary position in each obtained mode has a peak for each mode. I understand. The optical fiber loss measuring device of the present embodiment utilizes the fact that the Brillouin gain (Brillouin spectrum information) at an arbitrary position has a peak for each mode. For that matter, when considering FMF propagating in two modes for simplicity, if the pump light, each of the probe light has amplitudes in both LP ₀₁ and LP ₁₁ modes, the following [1] [3] has three spectral peaks.
[1] v _01-01 (interaction between pump light and probe light components between LP ₀₁ )
[2] v _01-11 (interaction between the probe light component of the pump light component and LP ₁₁ of LP _01, and the interaction between the probe light component of the pump light component and LP ₀₁ of LP ₁₁₎
[3] v _11-11 (interaction between pump light and probe light components between LP ₁₁ )

Here, the Brillouin gain G _B generated in the optical fiber when set in Brillouin frequency corresponding to the mode propagating in the frequency difference between the pump light and the probe light (x, y) is represented by the following formula (2).

In the above equation (2), x and y are coordinates in the cross section of the optical fiber, A _ij (x, y) is the intensity distribution of the acoustic mode, and _Ep (x, y) and E _s (x, y) are the pump lights, respectively. The intensity distribution of the probe light. In the equation (2), since the intensity distribution of the acoustic mode is excited by the interaction between the pump light and the probe light, it can be regarded as the same as the overlap of the electric field distribution.

On the other hand, the Brillouin gain changes according to the overlap of the electric field distributions of the pump light and the probe light. In other words, when the electric field distribution of the pump light and the probe light both changes in the longitudinal direction of the optical fiber, the overlapping relationship changes and the generated gain also changes, so that the loss for each mode can be obtained from the gain amount. Can not.

In the optical fiber loss measuring apparatus according to the present embodiment, in order to eliminate the change in the amount of gain generation due to the change in the state of overlap between the electric field distributions of the pump light and the probe light, the state of the electric field distribution such as the fundamental mode in the probe light is A symmetric mode is selected, and Brillouin gain is generated between the probe light and the pump light, which is a non-axisymmetric mode in which the electric field distribution can be changed. Note that this configuration is an example, and the pump light may select an axially symmetric mode.

FIG. 2 is a diagram illustrating the pump light and the probe light at the point A and their interaction. In FIG. 2, (a) is the shape of the electric field intensity distribution of the probe light at the point A in the optical fiber cross section, (b) is the shape of the electric field intensity distribution of the pump light at the point A in the optical fiber cross section, and (c) is (D) shows the shape of the electric field intensity distribution in the cross section of the optical fiber of the Brillouin gain at the point A, and FIG. In FIG. 2, regions where the intensity of the probe light and the intensity of the pump light are present are indicated by oblique lines and black, respectively, and for simplicity, the cross section of the optical fiber in the axially symmetric mode is described as a rectangular shape. However, in the case of an axially symmetric mode, the cross section of the optical fiber is actually circular.

Here, the intensities Ep and Er of the pump light and the probe light shown in FIGS. 2A and 2B can be expressed by equations (3) and (4).

At this time, since the pump light and the probe light interact as shown in FIG. 2 (c), the acquired Brillouin gain G has the state shown in FIG. 2 (d), that is, the relationship shown in equation (5).

According to equation (5), it can be seen that the Brillouin gain occurs in a region corresponding to the overlap with the intensity distribution of the probe light in the cross-sectional direction of the optical fiber.

Here, consider the gain at point B, which is different from point A in the longitudinal direction. FIG. 3 is a diagram illustrating pump light and probe light at a certain point B and their interaction. 3A shows the shape of the electric field intensity distribution of the probe light at the point B in the cross section of the optical fiber, FIG. 3B shows the shape of the electric field intensity distribution of the pump light at the point B in the cross section of the optical fiber, and FIG. The shape of the electric field intensity distribution at the point B in the cross section of the optical fiber, and the shape of the electric field intensity distribution at the point B in the cross section of the Brillouin gain optical fiber are shown. Also in FIG. 3, regions where the intensity of the probe light and the intensity of the pump light are present are shown by hatching and black, respectively, and for simplicity, the cross section of the optical fiber in the axially symmetric mode is described as a rectangular shape. However, in the case of an axially symmetric mode, the cross section of the optical fiber is actually circular.

Here, the intensities Ep and Er of the pump light and the probe light shown in FIGS. 3A and 3B can be expressed by the equations (6) and (7).

The electric field intensity distribution in the sectional direction of the probe light at the point B is the same as the point A because the electric field distribution is in a mode in which the state of the electric field distribution is axially symmetric. At this time, since the pump light and the probe light interact as shown in FIG. 3C, the Brillouin gain G has the state shown in FIG. 3D, that is, the relationship shown in Expression (8).

According to equation (8), the pump light and the probe light interact like point B, although the rotation state (interaction part in the optical fiber cross section) is different.

As described above, in the optical fiber loss measuring apparatus according to the present embodiment, by using the mode in which the state of the electric field distribution in the probe light is axially symmetric, even when the shape of the electric field distribution of the pump light changes due to propagation or the like, the same. The Brillouin gain can be obtained under the following conditions. That is, it is possible to generate Brillouin gain without depending on the shape of the electric field distribution of the pump light.

Also, since the electric field distribution of the probe light is axially symmetric, the polarization dependence of the Brillouin gain can be eliminated by applying a polarization scrambler and averaging the gain. Specifically, in the optical fiber loss measuring device shown in FIG. 1, a polarization modulation means for changing the polarization state of the probe light p2 is provided between the frequency control means 14 and the axially symmetric mode conversion means 16, so that the gain is increased. What is necessary is just to perform the process which averages the Brillouin gain of several polarization states in the analysis means 21. For example, a wave plate can be used as the polarization modulation means. When the polarization state of the probe light p2 is changed by the polarization modulation unit, the gain analysis unit 21 obtains the Brillouin gains of the plurality of changed polarization states, and averages the obtained Brillouin gains.

The state of the axially symmetric electric field distribution refers to a mode such as LP ₀₁ , LP ₀₂ , and LP ₀₃ in which the electric field distribution does not change even when rotated in the azimuthal direction of the cross section of the optical fiber. Further, the frequency difference between the pump light and the probe light incident on the optical fiber to be measured is set to a Brillouin frequency shift acting between different modes as shown in the above [2]. Thereby, it is possible to make the conditions of the overlap of the pump light and the probe light the same in the longitudinal direction of the optical fiber.

(Loss for each mode in the optical fiber f to be measured)
Next, the loss for each mode at each position in the longitudinal direction of the optical fiber f to be measured will be described. As an example, considering the loss measurement for each mode in the FMF, when the overlapping state is the same in the longitudinal direction, the Brillouin gain generated at the z point of the optical fiber can be expressed by the following equation (9).

In equation (9), g _B is a Brillouin gain coefficient, and I _P and I _S are integral values of the pump light and the probe light in the optical fiber cross-sectional area, respectively.

Here, consider the case where the coordinates of the pump light incident point are 0, the coordinates of the probe light incident point are L, the loss coefficient of the optical fiber is α, and the Brillouin gain generated at the point z ₁ is observed at the pump light incident point. , The observed Brillouin gain can be expressed by equation (10).

Similarly, the Brillouin gain at the point z ₂ can be expressed by equation (11).

Here, the loss distribution from a point z ₁ can be acquired by calculating the following equation (12). In the following equation (12), ln indicates a natural logarithm.

Thus, by analyzing the characteristics of the stimulated Brillouin scattered light, it is possible to measure the loss of the specific mode of FMF with a certain point z ₁ in the reference.

測定 The measuring method in the optical fiber loss measuring device of the present embodiment includes the following four steps.

First step: Set the frequency difference between pump light and probe light. The frequency difference is set so as to correspond to a Brillouin frequency shift corresponding to a combination of the modes of the pump light and the probe light incident on the optical fiber f to be measured.

(2) Second step: The mode of the pump light is converted into a mode whose loss is to be measured, and the mode of the probe light is converted into a mode in which the electric field distribution is axially symmetric in the optical fiber cross section.

Third step: The pump light and the probe light converted in the second step are incident on the optical fiber.

Fourth stage: Brillouin gain is obtained from the output of the probe light, and the loss is calculated.

According to the optical fiber loss measuring device of the present embodiment, even when the probe light is rotated in the azimuthal direction in the cross section of the measured optical fiber such as LP ₀₁ , LP ₀₂ , LP ₀₃ , the axis in which the electric field distribution does not change. The electric field distribution of the pump light, which is a non-axisymmetric mode, at an arbitrary position in the length direction of the optical fiber to be measured by converting into a symmetrical mode and entering the optical fiber to be measured to generate Brillouin gain. It is possible to non-destructively measure the loss received by the pump light even when the value changes. This makes it possible to judge the quality of the connection point, device, and the like in the transmission path of the optical fiber to be measured.

[Second embodiment]
FIG. 4 is a diagram illustrating an example of an optical fiber loss measuring device according to the second embodiment. In the optical fiber loss measuring apparatus according to the first embodiment, the pump light p1 is incident on one side of the optical fiber f to be measured and the probe light p2 is incident on the other side. In the fiber loss measuring device, the pump light p1 and the probe light p2 enter the optical fiber f to be measured from the same end. The pump light p1 and the probe light p2 are incident as pulses at different timings, and the pump light p1 or the probe light p2 is reflected by an optical reflector provided on the end face opposite to the incident end of the optical fiber to be measured. The two lights interact, and the intensity of the light output from the same end as the incident end is measured. The other configuration is the same as that of the first embodiment, and the description is omitted.

The optical fiber loss measuring device according to the present embodiment shown in FIG. 4 includes a pulse generator 13a, such that the light output from the laser light generating means 11 and divided into two by the branching element 12 is pulsed at a predetermined timing. 13b and timing control means 22a and 22b are provided. The two lights pulsed at a predetermined timing by the

pulse generators

13a and 13b and the timing control means 22a and 22b are processed by the mode demultiplexing means 15, the frequency control means 14 and the axially symmetric mode conversion means 16, respectively. The light is multiplexed by the multiplexing element 23, and is input from one end of the optical fiber f to be measured by the optical circulator 17 as the pump light p1 and the probe light p2.

The

pulse generators

13a and 13b and the timing control means 22a and 22b operate so that the pump light p1 and the probe light p2 are incident on the measured optical fiber f with an incident time difference corresponding to each position of the measured optical fiber f. The two lights are pulsed with the timing shifted.

In the optical fiber loss measuring device of the present embodiment shown in FIG. 4, an optical reflector 24 is provided at the other end of the optical fiber under test f opposite to the end connected to the optical circulator 17. I have. Since the pump light p1 and the probe light p2 incident from one end of the optical fiber f to be measured are incident at different timings, the light reflector 24 reflects the light incident first. The reflected light has its traveling direction reversed, and as a result of the two lights propagating oppositely in the measured optical fiber f, they collide with each other and cause an interaction that causes a Brillouin gain. Since the Brillouin gain generated by the interaction is superimposed on the Brillouin scattered light s output from one end of the optical fiber f to be measured, by observing this Brillouin gain, each mode of the optical fiber f to be measured can be determined. Loss can be measured.

In the optical fiber loss measuring device of the present embodiment, in order to obtain a loss distribution from the generated Brillouin gain, it is necessary to perform one measurement while changing the time difference of incidence. Specifically, acquiring the gain generated at one end of the optical fiber f to be measured, repeating the acquisition of the gain again by shifting the collision position by slightly changing the incident time difference, and finally obtaining the optical fiber to be measured By obtaining the Brillouin gain up to the other end of f, the loss distribution is obtained from the Brillouin gain at each position of the fiber.

In the optical fiber loss measuring device of the present embodiment, one measurement of the gain results in a state in which the optical fiber to be measured contains one pump light p1 and one probe light p2. Since the output timing is different between the pulse of the probe light p2 (Brillouin scattered light s) on which the Brillouin gain is superimposed and the pulse of the pump light p1 that has returned after reflection, the Brillouin is adjusted by adjusting the measurement timing of the output light. It is possible to measure the gain.

The configuration of the optical fiber loss measuring device of the embodiment described above is an example, and similarly, an optical frequency difference corresponding to a frequency (wavelength) shift is given between the pump light and the probe light to excite an arbitrary propagation mode. Any means can be used as long as it is a device configuration capable of extracting the amplified probe light in the time domain. Also, in a general SMF, since the configuration of the optical fiber loss measuring device of the present embodiment can be applied by shortening the incident wavelength, the optical fiber to be measured has a condition in which multiple modes propagate. Should be fine.

DESCRIPTION OF SYMBOLS 11 Laser light generation means 12 Branch element 13 Pulse generator 14 Frequency control means 15 Mode demultiplexing means 16 Axisymmetric mode conversion means 17 Optical circulator 18 Photoelectric conversion means 19 A / D conversion means 20 Data extraction means 21 Gain analysis means 22a, 22b Timing control means 23 Multiplexing element 24 Optical reflector f Optical fiber to be measured

Claims

Light having a first frequency of a predetermined mode is incident as pump light on an optical fiber to be measured that propagates a plurality of modes, and corresponds to a Brillouin frequency shift of the predetermined mode from the first frequency. Light incidence means for entering light of a second frequency lower by the frequency as probe light,
Brillouin gain acquisition means for measuring the intensity of light output from the measured optical fiber and acquiring Brillouin gain in the longitudinal direction of the measured optical fiber by a BOTDA (Brillouin Optical Time Domain Analysis) method;
Means for measuring the loss of the measured optical fiber in the predetermined mode by comparing the magnitudes of the Brillouin gains at respective positions in the longitudinal direction of the measured optical fiber. A loss measuring device,
The optical fiber loss measuring device, wherein the probe light is in a mode in which an electric field distribution in a cross section of the optical fiber to be measured is axially symmetric regardless of a mode of the pump light.
The light incident means receives the pump light from one end of the optical fiber to be measured and the probe light from the other end of the optical fiber to be measured, and the Brillouin gain obtaining means comprises: The optical fiber loss measuring device according to claim 1, wherein the intensity of light output from the other end of the target optical fiber is measured.
The optical fiber further comprises a light reflecting means provided at an end of the optical fiber to be measured opposite to a light incident end, wherein the light incident means pulsates the pump light and the probe light, and then sets the The incident light from one end of the fiber at a different timing, and the Brillouin gain acquisition unit measures the intensity of light reflected from the light reflection unit and output from one end of the optical fiber to be measured. 2. The optical fiber loss measuring device according to 1.
Polarization modulation means for changing the polarization state of the probe light incident from the other end of the optical fiber to be measured, and averaging calculation means for averaging the Brillouin gain when the polarization state is changed. The optical fiber loss measuring device according to any one of claims 1 to 3, further comprising:
The optical fiber according to any one of claims 1 to 4, wherein the predetermined mode of the pump light is a mode in which an electric field distribution in a cross section of the optical fiber to be measured is non-axially symmetric. Loss measuring device.
The non-axisymmetric mode rotation mechanism for rotating an electric field distribution of a cross section of the optical fiber under test of the pump light incident from the other end of the optical fiber under test, further comprising: The optical fiber loss measuring device according to claim 1.
Light having a first frequency of a predetermined mode is incident as pump light on an optical fiber to be measured that propagates a plurality of modes, and corresponds to a Brillouin frequency shift of the predetermined mode from the first frequency. A light incident step in which light of a second frequency lower by the frequency is incident as probe light;
A Brillouin gain acquiring step of measuring the intensity of light output from the measured optical fiber and acquiring the Brillouin gain in the longitudinal direction of the measured optical fiber by a BOTDA (Brillouin Optical Time Domain Analysis) method;
Measuring the loss in the predetermined mode of the measured optical fiber by comparing the magnitudes of the Brillouin gains at respective positions in the longitudinal direction of the measured optical fiber. A measuring method,
An optical fiber loss measuring method, wherein the probe light is a mode in which an electric field distribution in a cross section of the optical fiber to be measured is axially symmetric regardless of a mode of the pump light.